Immunological, biochemical and morphological studies are proposed to identify cell surface constituents specific to purified populations of mouse spermatogenic cells, from primitive type A spermatogonia to mature spermatozoa. Previous studies in this laboratory have identified antigenic determinants present only on the surface of late germ cells in the testis. Current investigations are aimed at the biochemical analysis of male germ cell membranes. Accordingly, procedures have been developed for the fractionation of pure cell surface membranes from pachytene spermatocytes, round spermatids, and residual bodies. Polyacrylamide gel electrophoretic analysis indicates that substantial changes occur in the polypeptide composition of germ cell membranes as spermatogenesis proceeds. Experiments utilizing sugar-binding lectins have allowed the identification of cell surface glycoproteins of developing spermatogenic cells. Finally, enzyme marker analysis of male germ cells indicates that somatic cells and germ cells differ significantly in their molecular organization. In toto, these studies provide the first opportunity for a detailed description of the physiological role of cell surfaces mammalian spermatogenesis.